1. Field of the Invention
The present invention relates to an image-sensing apparatus having a solid-state image-sensing device that can convert incident light into an electric signal selectively either linearly or logarithmically, and particularly to an image-sensing apparatus having a single image-sensing device whose operation can be switched between linear conversion and logarithmic conversion.
2. Description of the Prior Art
Conventionally, a solid-state image-sensing device, such as an area sensor, having photosensitive devices, such as photodiodes, arranged in a matrix outputs a signal that is obtained by linearly converting the brightness of the light incident on those photosensitive devices. An area sensor that performs such linear conversion (an area sensor of this type will hereafter be referred to as a “linear sensor”) is adjusted, for example by adjusting the aperture of a lens, in such a way that those of the photosensitive devices which are sensing the brightest parts (highlights) of a subject output an electric signal having about 90% of the maximum level that they can output. By using a linear sensor like this, it is possible to acquire the data of a subject with superb gradation as long as, assuming that the brightness of the subject distributes from a minimum value Lmin [cd/m2] to a maximum value Lmax [cd/m2], the brightness range of the subject Lmax/Lmin is so narrow as to be expressed with a two-digit value.
On the other hand, in U.S. Pat. No. 5,241,575, the assignee of the present invention once proposed an area sensor provided with a photosensitive device that outputs a photoelectric current in proportion to the amount of incident light, a MOS transistor to which the photoelectric current is fed, and a bias means for biasing the MOS transistor in such a way that a subthreshold current flows therethrough, so that the photoelectric current is converted logarithmically (an area sensor of this type will hereafter be referred to as a “LOG sensor”). A LOG sensor like this, by being adjusted in the same manner as with a linear sensor, i.e. in such a way that those of its photosensitive devices which are sensing the brightest parts (highlights) of a subject output an electric signal having about 90% of the maximum level that they can output, permits the data of the subject to be acquired in a brightness range Lmax/Lmin so wide as to be expressed with a five- to six-digit value.
However, a linear sensor permits image sensing in a brightness range so narrow as to be expressed with a two-digit value. Accordingly, when the brightness of a subject is high, as in a case where the subject is in direct sunlight, the brightness of bright parts of the subject exceeds the level that the photosensitive devices can handle, and thus causes an overflow in them. This makes it impossible to acquire the data of parts of the subject where the brightness exceeds that level, and thereby causes saturation. Saturation can be avoided by shifting the brightness range in which data acquisition is feasible into a brighter region to make it possible to acquire the data of bright parts of the subject; however, this in turn makes it impossible to acquire the data of dim parts of the subject, and thus leads to flat blackness.
On the other hand, a LOG sensor offers a logarithmic output characteristic as shown in FIG. 36. Therefore, using a LOG sensor tends to result in poor gradation in bright parts of a subject. Accordingly, for example, whereas a LOG sensor permits acquisition of both dim and bright parts of a brightly-lit subject, it yields rather poor gradation in bright parts of a dimly-lit subject.
Given the characteristics of a linear sensor and of a LOG sensor described above, it will be understood that a linear sensor is effective in shooting a subject having a narrow brightness range and that a LOG sensor is effective in shooting a subject having a wide brightness range. Accordingly, a linear sensor is effective in shooting a subject while zooming in on it or in shooting a subject at a close distance, because in such cases the shooting range is narrow and thus, generally, the brightness range is also narrow. By contrast, a LOG sensor is effective in shooting a subject at a far distance without zooming in on it, particularly when shooting it outdoors under a clear sky, because in such cases the shooting range is wide and thus the brightness range is also wide.